General access page intercom systems are in widespread use in industrial plants throughout the United States. These system permit page announcements throughout a facility of necessary information to the facility personnel. The page intercom system is frequently used to provide general information to personnel, to request personnel to answer on specified telephone type intercoms, or in emergency situations to request assistance or to provide instruction.
Persons making false announcements, cat calls, or other abusive noises on an intercom system annoy and possibly endanger other personnel by interrupting emergency communications. A familiar example of abusive communications is the transmission of "cat calls" during emergency drills at a nuclear facility. In addition to disrupting the drill, additional expense is incurred by the facility when the drill is required to be repeated due to the interruptions. Such abusive uses of the intercom system lower the prospect for safe operation of the facility. Many more examples of abusives calls are commonly experienced in all types of industries.
To minimize abuse of a page intercom system, it is desirable to identify the station where the abusive call originates. In a central exchange telephone system, identification of the station of origin is easily accomplished since each station is connected to a dedicated or private line. The source of a communication can be identified by simply monitoring wire usage. In party line systems, however, all stations are connected to a common party line. Therefore, it is not possible to identify the station of origin by monitoring the party line.